FWIW my GTI ('05 on a 55 plate) has just hit 100k miles, 80% on motorway cruising at 80mph (indicated, 70mph true officer ;-) ) I fitted ITG, HPFP, TBE miltek and stage 2+ map at 90k miles and it made 275BHP and 315lbft torque on the rollers at 90k miles, so pretty good fels just run in :-) :-) In 100k miles it has had a air con pump sieze, has a small weep from the oil cooler, which I must get round to fixing and has had to have a drive shaft boot replaced, otherwise in great running order. If the engine is warmed up every run and then not stressed, the only issue with high miles will be the suspension. I had a Citroen 1.9i Volcane (Peugeot 309 GTI chassis and engine) but the suspension was like a tin tray full of marbles at 160k miles, the golf is a very different beast - German engineering :-)
By contrast, my neighbour had a 997 2S that only ever did track days and was smoking on start up like a steam engine after 7k track day miles!! (and not just from the usual flat six oil collecting in head on standing). He